1. Field of the Invention
The present invention relates to channel add/drop filters and channel monitors employing photonic crystals, and in particular to improvements in the adaptability of such channel add/drop filters and channel monitors to polarized waves.
It should be understood that in the present specification, the significance of the term “light” is meant to also include electromagnetic waves that relative to visible light are of longer as well as shorter wavelength.
2. Description of the Background Art
Along with advances in wavelength division multiplexed optical communication systems in recent years, the importance of optical devices such as add/drop filters, channel filters, and channel monitors is on the rise. In addition, miniaturization of these types of optical devices is being sought. If for example a wavelength monitor were to be provided for each of optical amplifiers/transponders in an optical communication system, the wavelength monitor would have to be installed on the optical transponder platform. But because wavelength monitors currently being used are large, installing them on the platforms is physically impossible. Thus owing to the need for miniaturization of optical devices, photonic crystals are being used in attempts to develop extraordinarily small-scale optical devices. In particular, with photonic crystals extremely small-scale optical devices can be realized by exploiting artificial periodic structures in which a crystal-lattice-like periodic, ultra-dense refractive index distribution is artificially imparted within the parent material.
One important feature of photonic crystals is the presence of photonic bandgaps. With photonic crystals having three-dimensional refractive index periodicity (3D photonic crystals), perfect bandgaps in which the transmission of light is prohibited in every direction can be formed. Among the possibilities with these crystals are the local confinement of light, control of spontaneous emission, and formation of waveguides by the introduction of line defects, wherein the realization of ultrasmall photonic integrated circuits is anticipated.
Meanwhile, studies into uses for photonic crystals having a two-dimensional periodic refractive-index structure (2D photonic crystals), are flourishing because the crystals can be manufactured comparatively easily. A periodic refractive-index structure in 2D photonic crystals can be formed by, for example, arranging in a square-lattice or triangular-lattice geometry air rods perforating a high-refractive-index plate material (usually termed a “slab”). Alternatively the structure can be formed within a low-index material by arranging, in a 2D-lattice geometry within it, posts made of a high-refractive-index material. Photonic bandgaps can be produced from such periodic refractive-index structures, enabling the transmission of light traveling in an in-plane direction (direction parallel to both the principal faces of the slab) to be controlled. Waveguides, for instance, may be created by introducing line defects into a periodic refractive-index structure. (See, for example, Physical Review B, Vol. 62, 2000, pp. 4488–4492.)
FIG. 5 illustrates, in a schematic oblique view, a channel add/drop filter disclosed in Japanese Unexamined Pat. App. Pub. No. 2001-272555. (In the drawings in the present application, identical reference marks indicate identical or equivalent parts.) The channel add/drop filter in FIG. 5 exploits a 2D photonic crystal having, configured within a slab 1, cylindrical through-holes 2 of identical diameter (ordinarily occupied by air) formed at the vertices of a 2D triangular lattice. In a 2D photonic crystal of this sort, light is prohibited from propagating in an in-plane direction within the slab 1 by a bandgap, and in the direction normal to the plane (direction orthogonal to the two principal faces of the slab) is confined due to total internal reflection occurring at the interface with the low-refractive-index clad (air, for example).
The photonic crystal in FIG. 5 contains a waveguide 3 consisting of a straight line defect. This straight-line defect 3 includes a rectilinearly ranging plurality of lattice points adjoining each other, with the through-holes 2 missing in these lattice points. With light being able to propagate through a defect in a 2D photonic crystal, the straight-line defect can be employed as a linear waveguide. With linear waveguides, the spectrum of wavelengths in which light can be transmitted at low loss is comparatively broad; consequently light in a wide range of wavelength containing signals in a plurality of channels may be propagated through them.
The photonic crystal set out in FIG. 5 also contains a cavity 4 consisting of a point defect. The point defect 4 contains a single lattice point, and through that lattice point a through-hole that is of large diameter as compared with the other lattice points is formed. A defect in this way containing a relatively large-diameter through-hole is generally termed an acceptor-type point defect. On the other hand, a defect in which through-holes are missing in lattice points is generally termed a donor-type point defect. The cavity 4 is disposed adjacent the waveguide 3, within a range in which they can exert on each other an electromagnetically reciprocal effect.
In a 2D photonic crystal such as mat illustrated in FIG. 5, if light 5 containing a plurality of wavelength ranges (λ1, λ2, . . . λi, . . . ) is introduced into the waveguide 3, light that has the specific wavelength λi corresponding to the resonant frequency of the cavity 4 is trapped in the cavity and while resonating in the interior of the point defect, light 6 of wavelength λi is emitted in the plane-normal direction, in which due to the finite thickness of the slab 1 the Q factor is small. This means that the photonic crystal in FIG. 5 can be employed as a channel drop filter. Conversely, by shining light into the point defect 4, in the direction normal to the slab 1, light of wavelength λi that resonates within the cavity 4 can be introduced into the weveguide 3. This means that the photonic crystal in FIG. 5 can also be employed as a channel add filter. It will be appreciated that the transfer of light between either the waveguide 3 or the cavity 4 and the exterior can be made to take place by proximately disposing an optical fiber or en optoelectronic transducer in the vicinity of the waveguide end faces or the vicinity of the cavity. Of course, in that case a collimating lens (collimator) may be inserted in between either the waveguide end face or the cavity, and the optical-fiber end face or the optoelectronic transducer.
In an optical add/drop filter such as that illustrated in FIG. 5, by appropriately configuring the spacing between the waveguide 3 consisting of the line defect and the cavity 4 consisting of the point defect, the ratio of optical intensities in a specific wavelength that is transferred between the waveguide and the cavity can be controlled. Also in FIG. 5, since no asymmetry is introduced with respect to the point defect 4 in the direction normal to the slab 1, light is output in both vertical directions from the point defect 4; but it is possible to make the output of light be in only one or the other vertical direction by introducing asymmetry in the point defect 4 in the plane-normal direction. An example of a mechanism that can be utilized to introduce this sort of asymmetry is a method in which the diameter of the point defect 4, which is round in section, is made to vary continuously or discontinuously along the thickness of the slab. With further regard to FIG. 5, although the channel add/drop filter in the figure contains only a single cavity, it will be readily understood that by disposing along the waveguide a plurality of cavities differing from one another in resonant wavelength, optical signals in a plurality of channels can be added/dropped. It will be appreciated that the resonant wavelength of the cavity 4 can be changed by, for example, altering the dimensions/shape of the point defect.
The fact that, as described above, a channel add/drop filter such as that depicted in FIG. 5 makes it possible to extract as light 6 light of a specific wavelength λi only—contained within an optical signal 5—via the cavity 4 means that the filter may be employed in wavelength monitors.
Reference is made to FIG. 6, which schematically illustrates in an oblique view one example of a wavelength monitor employing a 2D photonic crystal such as just discussed. Three cavities 4a, 4b and 4c differing from one another in resonant frequency are provided in this wavelength monitor, and adjacent to these cavities the end faces of optical fibers 10a, 10b and 10c are disposed so as to admit the light of the specific frequencies radiated from the cavities. These optical fibers are in turn connected to photoelectric sensing elements (not illustrated), wherein the specific frequencies of light are detected by the photoelectric sensing elements.
Nevertheless, in a channel add/drop filter employing a 2D photonic crystal such as illustrated in FIG. 5, within the light of the specific wavelength λi only that portion whose electric-field vector has a component parallel to the principal plane of the 2D photonic crystal 1 can be extracted from the cavity 4 as emitted light 6. On the other hand, the light 5, which is introduced into the waveguide 3 by, for example, an optical fiber, will at times be polarized in a specific direction by the optical fiber or by the impact of the environment leading up to it. For instance, a situation where the electric-field vector of the light of wavelength λi contained in the introduced light 5 is polarized perpendicular to the principal plane of the 2D photonic crystal 1 will mean that light of wavelength λi cannot be monitored using the channel add/drop filter of FIG. 5. Likewise too, in a situation where the electric-field vector of the light of wavelength λi is polarized so as to be inclined with respect to the principal plane of the 2D photonic crystal 1, since within the light only that portion that has an electric-field-vector component parallel to the principal plane of the 2D photonic crystal 1 is what can be monitored within the light of wavelength λi using the channel add/drop filter of FIG. 5,the proportional intensity of the light of wavelength λi contained in the introduced light 5 cannot be monitored correctly.